Crosstalk (or inter-channel interference) is a major source of channel impairment for Multiple Input Multiple Output (MIMO) communication systems, such as Digital Subscriber Line (DSL) communication systems.
As the demand for higher data rates increases, DSL systems are evolving toward higher frequency bands, wherein crosstalk between neighboring transmission lines (that is to say, transmission lines that are in close vicinity such as twisted copper pairs in a cable binder) is more pronounced (the higher frequency, the more coupling).
A MIMO system can be described by the following linear model:Y(k)=H(k)X(k)+Z(k)  (1)wherein the N-component complex vector X, respectively Y, denotes a discrete frequency representation, as a function of the frequency/carrier/tone index k, of the symbols transmitted over, respectively received from, the N channels,wherein the N×N complex matrix H is referred to as the channel matrix: the (i,j)-th component of the channel matrix H describes how the communication system produces a signal on the i-th channel output in response to a symbol being transmitted to the j-th channel input; the diagonal elements of the channel matrix describe direct channel coupling, and the off-diagonal elements of the channel matrix describe inter-channel coupling (also referred to as crosstalk coefficients),and wherein the N-component complex vector Z denotes additive noise over the N channels, such as Radio Frequency Interference (RFI), thermal noise and alien interference.
Different strategies have been developed to mitigate crosstalk and to maximize effective throughput, reach and line stability. These techniques are gradually evolving from static or dynamic spectral management techniques to multi-user signal coordination (or vectoring).
One technique for reducing inter-channel interference is joint signal precoding: the transmit data symbols are jointly passed through a precoder before being transmitted over the respective communication channels. The precoder is such that the concatenation of the precoder and the communication channel results in little or no interference at the receiver. Typically, the precoder performs a matrix-product of a row-vector of frequency samples to be jointly transmitted over multiple channels with a precoding matrix so as to compensate for an estimate of the coming crosstalk.
A further technique for reducing inter-channel interference is joint signal post-processing: the received data symbols are jointly passed through a postcoder before being detected. The postcoder is such that the concatenation of the communication channel and the postcoder results in little or no interference at the receiver. Typically, the postcoder performs a matrix-product of a row-vector of frequency samples jointly received from multiple channels with a crosstalk cancellation matrix so as to cancel an estimate of the incurred crosstalk.
Signal precoding is particularly suited for downstream communication (toward customer premises), while signal post-processing is particularly suited for upstream communication (from customer premises). Either technique is often referred to as signal vectoring.
Signal vectoring is typically performed at a traffic aggregation point, where all the data symbols concurrently transmitted over, or received from, multiple communication channels are available. For instance, signal vectoring is advantageously performed within a Digital Subscriber Line Access Multiplexer (DSLAM).
The choice of the vectoring group, that is to say the set of communication lines, the signal s of which are jointly processed, is rather critical for achieving good crosstalk cancellation performances. Within a vectoring group, each communication line is considered as a disturbing line inducing crosstalk into the other communication lines of the group, and the same communication line is considered as a victim line receiving crosstalk from the other communication lines of the group. Crosstalk from lines that do not belong to the vectoring group is treated as alien noise and is not canceled.
Ideally, the vectoring group should match the whole set of communication lines that physically and noticeably interact with each other. Yet, local loop unbundling on account of national regulation policies and/or limited vectoring capabilities may prevent such an exhaustive approach, in which case the vectoring group would include a sub-set only of all the physically interacting lines, thereby yielding limited crosstalk cancellation performances.
The performance of signal vectoring depends critically on the component values of the precoding or cancellation matrix, which component values are to be computed and updated according to the actual and varying crosstalk couplings.
A prior art method for estimating the crosstalk coefficients comprises the steps of:                simultaneously transmitting a plurality of mutually orthogonal crosstalk pilot sequences through respective ones of a plurality of disturber channels,        measuring errors induced over a victim channel while the pilot sequences are being transmitted,        correlating the error measurements with respective ones of the plurality of crosstalk pilot sequences, thereby yielding a plurality of correlated error measurements,        estimating the crosstalk coefficients from the plurality of disturber channels into the victim channel based on respective ones of the plurality of correlated error measurements.        
That is, transceiver units send downstream or upstream pilot sequences. Error samples, measuring both interference and noise over the victim channel, are fed back to a vectoring controller. The error samples are correlated with a given pilot sequence in order to obtain the crosstalk contribution from a specific line. To reject the crosstalk contribution from the other lines, the pilot sequences are made orthogonal, for instance by using Walsh-Hadamard sequences comprising ‘+1’ and ‘−1’ anti-phase symbols. The crosstalk estimates are used for updating the precoding or cancellation matrix. The process can be repeated as needed to obtain more and more accurate estimates.
This prior art method has been adopted by the International Telecommunication Union (ITU) for use with VDSL2 transceivers, and is described in the recommendation entitled “Self-FEXT Cancellation (vectoring) For Use with VDSL2 Transceivers”, ref. G.993.5 (04/2010).
In this recommendation, the pilot signals are sent on the so-called SYNC symbols, which occur periodically after every 256 DATA symbols.
On a given disturber line, a representative subset of the active carriers (or tones) of the SYNC symbol are 4-QAM modulated by the same pilot digit from a given orthogonal pilot sequence, and thus all transmit one of two complex constellation points, either ‘1+j’ corresponding to ‘+1’, or ‘−1−j’ corresponding to ‘−1’. The remaining carriers of the SYNC symbol keeps on carrying the typical SYNC-FLAG for On-Line Reconfiguration (OLR) message acknowledgment.
On a given victim line, error samples, which comprise both the real and imaginary part of the slicer error on a per tone or group-of-tones basis, quantized with a certain number of bits (typically 16), are measured and reported for a specific SYNC symbol to the vectoring controller for further crosstalk estimation.
In G.993.5, it is further assumed that the access node transmits and receives the SYNC symbols over the vectored lines synchronously (super frame alignment) so as pilot signal transmission and interference measurements are carried out synchronously over the respective transmission lines.
Also, a joining line confines transmission to SYNC symbols only during a preliminary initialization phase (O-P-VECTOR1) so as not to impair the communication over the active lines (no data is being transmitted during the SYNC symbols), yet to allow the system to acquire the crosstalk coefficients, and to cancel the crosstalk, from that new joining line towards the already active lines. Then, the joining line can switch to full transmission once the crosstalk coefficients from that new joining line have been determined and the precoder and postcoder have been updated to mitigate the crosstalk from that new joining line.
A further prior art method for estimating the crosstalk coefficients is described in Appendix III of ITU G.993.5 recommendation, and is particularly suited for legacy Customer Premises Equipment (CPE).
The method comprises the steps of superimposing a crosstalk probing signal over the victim line while measuring signal to Noise and Interference Ratio (SNIR) over that victim line, and estimating the crosstalk coefficients from these SNIR measurements.
The probing signals consists of a complex-weighted sum of regular data signals transmitted over the respective disturber lines. Three successive SNIR measurement rounds with three distinct complex weight values are required so as to estimate both the amplitude and phase of the crosstalk coefficients from a given disturber line.
The basic SNIR method uses successive perturbations on the victim lines, followed by successive cancellation. Due to slow reporting times of SNIR measurements of up to 10 seconds per measurement round, the overall time needed to acquire the crosstalk coefficients for a joining legacy line in a scenario where many lines are active, is significant (e.g., for 48 lines one iteration may take about 24 minutes, and multiple iterations are generally needed to obtain accurate estimates).
A solution is sought to rapidly acquire the crosstalk coefficients in a DSL communication system, in particular for situations where vectoring-compliant and legacy Customer Premises Equipment (CPEs) co-exist, and where one or more activating (or initializing) lines join a large number of active lines. It is essential to quickly attain vectored performance for both vectoring-compliant and legacy CPEs. More generally, we are concerned with fast and accurate acquisition of crosstalk coefficients in a wired multi-carrier communication system from a set of disturber lines into a victim line.